1. Field of the Invention
The present invention relates to a transmitter having a power supply, and more particularly to a method and an apparatus for generating a transmitting wave without deteriorating transmission characteristics due to a reduction in a power supply voltage.
2. Description of the Related Art
In mobile communication equipment, a reduction in power supply voltage adversely influences the radio transmission characteristics. Especially, to almost all the digital modulations in practice such as 16QAM, pi/4 QPSK and a multi-carrier 16QAM having an amplitude modulation component, the reduction in power supply voltage may lead to significant problems, for example, communication impossibility.
A general circuit configuration of a digital modulation transmitter is shown in FIG. 1. A modulating signal generator 1 generates a modulating signal Sm on the basis of transmitting data, and a digital modulator 2 modulates a carrier wave in accordance with the modulating signal Sm. A modulated wave St is amplified by a transmission power amplifier 3 and is transmitted to the air through an antenna. Power is supplied from a constant-voltage power supply 4 to the transmission power amplifier 3.
Almost all the digital modulations in practice have an amplitude modulation component, and a momentary maximum value (a peak value) thus exists in the transmitting wave. Therefore, the transmission power amplifier 3 requires a power which can cope with this peak value. In the 16QAM, for example, since the transmission power amplifier 3 requires 10Watts of power in order to obtain 1 Watt of an average transmitting power, a larger power is necessary to deal with the peak value.
When the power supply voltage of the transmitter is lowered to a value which the constant-voltage power supply 104 can not deal with, however, the power amplifier 3 can not output the peak value of the transmission output. In such a case, an error rate slightly deteriorates only if the amplitude component is just restricted, but the communication system is significantly influenced by increasing in the noise from outside a channel (or the leakage power to an adjacent channel and the like) generated by a phase error or a distortion which is caused due to the passage having non-linear characteristics. When using a feedback loop in a transmission control system, there is a possibility such that the loop is opened at the time of the peak to involve a control failure, leading to defective communication in some cases.
In order to solve such a problem, the amplitude component must be controlled before the peak value is restricted by the power amplifier so that no distortion or the like of the transmission output is generated. If the amplitude component is tried to be controlled by using the modulated wave St, since there exists a limitation such that a filter for an extremely narrow bandwidth is necessary, this is not a realistic solution.
Further, there have been proposed several methods for controlling the power supplied to a power amplifier. For example, in Japanese patent laid-open publication No. Sho 57-140008, an audio power amplifying circuit is disclosed which controls the output voltage of the variable-voltage power supply in accordance with a detected peak level of an output signal. In Japanese patent laid-open publication No. Sho 58-24056, a mobile station is disclosed which is provided with a current controller for controls current supplied to a transmission power amplifier depending on whether a modulating signal is received or not.
However, the above conventional methods are intended to save the consumption power or improve the effective use of the same but do not suggest a configuration for eliminating the above-described problem.